1. Field of the Invention
This invention relates to indexable cutter inserts for a rotary cutter such as a ball end mill.
2. Prior Art
In recent years, rotary cutters of the type having one or more cutter inserts of cemented carbide releasably attached to a cutter body have been extensively used. FIGS. 1 and 2 show one such conventional ball end mill 138 as disclosed in Japanese patent application Laid-Open (Kokai) No. 58-181502. A conventional cutter insert 110 of the indexable type employed in this ball end mill comprises a plate-like body 111 of a generally circular shape defined by a front face 112, a rear face 114 and a circumferential face 116. Three notches 120 are formed in the circumferential face 116 of the insert body 111 in circumferentially equally-spaced relation to one another to provide three circumferentially equally-spaced side faces 118 interrupted by the notches 120. Each of the notches 120 has a generally trapezoidal shape when viewed in a direction perpendicular to the front face 112 of the insert body 111, and is defined by a pair of first and second opposed walls 120a and 120b converging radially inwardly and a bottom wall 120c interconnecting the first and second walls 120a and 120b at their inner ends. The front face 112 of the insert body 111 cooperates with the three side faces 118 to define three cutting edges 122 of an equal length, so that each cutting edge 122 extends between the first wall 120a of one of each two adjacent notches 120 and the second wall 120b of the other. And, the cutting edge 122 has a leading end 122a where it intersects the first wall 120a of the one notch 120 and also has a trailing end 122b where it intersects the second wall 120b of the other notch 120. A substantially equilateral triangle is defined by three straight lines each extending between each adjacent ones of the leading ends 122a of the three cutting edges 122. The first wall 120a of each notch 120 is disposed in a plane 130 perpendicular to the front face 114 of the insert body 111 and passing through the leading end 122a of the cutting edge 122 and a center 131 of a circle along which the arcuate cutting edge 122 extends when viewed in a direction perpendicular to the front face 114.
A marginal portion 124 of the front face 112 disposed adjacent to and extending along each cutting edge 122 serves as a rake surface. The rake surface 124 is recessed adjacent to the first wall 120a of the trapezoidal notch 122 to provide a convexly curved surface 128. The rake surface 124 is also sloping toward the rear face 114 toward the trailing end 122b of the cutting edge 122. A straight second cutting edge 132 which extends from the trailing end 122b of each arcuate cutting edge 122 is also defined by the front face 112 and an outer portion of the second wall 120b of the trapezoidal notch 120. An angle .gamma. between the second cutting edge 132 and a line tangent to the arcuate cutting edge 132 at the trailing end 122b is obtuse (FIG. 8). An inner portion of the second wall 120b serves as a detent wall as later described.
As shown in FIGS. 1 and 2, the cutter insert 110 is removably secured by a clamp screw 139 to an insert pocket 140 of a body 138a of the ball end mill 138, with the rear face 114 mated with a seat surface 140a of the insert pocket 140, in such a manner that an indexed one of the three cutting edges 122 is so disposed as to extend outwardly beyond the front end and outer periphery of the end mill body 138a, the body 138a being adapted for rotation about an axis 0 thereof. A lock projection 142 on the insert pocket 140 is received in the trapezoidal notch 120 disposed between the cutting edges 122 other than the indexed cutting edge 122 with the inner detent portion of the second wall 120b - held against the lock projection 142, thereby preventing the insert body 111 against angular movement about the clamp screw 139. As shown in FIG. 1, the axial rake angle for the indexed cutting edge 122 is positive. The second cutting edge 132 extending from the indexed cutting edge 122 serves to cut a workpiece when it is retracted or moved rearwardly with respect to the workpiece during the cutting operation.
As described above, the first wall 120a of the trapezoidal notch 120 is disposed in the plane 130 passing through the leading end 122a of the cutting edge 122 and the center 131 of the circle along which the arcuate cutting edge 122 extends. And, the rake surface 124 has the convexly arcuate leading end portion 128. As a result, the effective length of the cutting edge 122 is reduced, and therefore a radius of cutting of the rotating end mill 138 is correspondingly reduced. In addition, as shown in FIG. 3, an angle .theta. defined by the first wall 120a of the trapezoidal notch 120 and a straight line tangent to the indexed cutting edge 122 at its leading end 122a is 90.degree., and therefore a corner portion defined by the first wall 120a and the side face 116 is susceptible to damage or chipping.